As used herein, the meaning of vitreous coating is a coating having a glass or enamel appearance which may be either organo-mineral, or totally mineral.
As used herein, the meaning of organo-mineral vitreous is a coating composed of a sol-gel type material (that is, obtained through a sol-gel treatment), whose network is substantially inorganic, yet including an organic groups, particularly because of the curing temperature of the coating and the precursors used.
As used herein, the meaning of entirely mineral coating is a coating composed of an entirely inorganic material, free from all organic groups. Such a coating may also be obtained through a sol-gel process with a curing temperature of at least 400° C., or from tetraethoxisilane (TEOS) precursors with a curing temperature which may be less than 400° C.
In the field of vitreous, non stick coatings, intended for use in culinary articles, sol-gel coatings, particularly those obtained from silica (silane) based, or alumina based (aluminate) metal alkoxides.
These coatings are currently experiencing wide development in the field of cooking items, in that they make it possible to obtain colored coatings with a particularly good resistance to temperature and scratches.
However, such coatings have limited adhesion on metal substrates, in particular, aluminum substrates, stainless steel substrates or cast iron substrates.
To alleviate these drawbacks, it has been known by the skilled person to prepare the substrate surface through chemical treatment (for example, a chemical etching treatment) or mechanical treatment (for example a brushing or sand blasting treatment), or even a combination thereof.
Nevertheless, even when such treatments are carried out, the adhesion of the non stick coating to the substrate remains limited, particularly when the non stick coating is subjected to a mechanical deformation such a an impact or drilling, for example, in order to fix a rivet or an assembling stud.
Thus, when cooking items, whose bottom has an internal side coated with a sol-gel type, non stick coating, are subjected violent shocks, a mark is observed under the impact, accompanied with cracks propagating radially even though the internal side of the bottom has been brushed or sand blasted beforehand.
Thus, the manufacturing of such cooking items should be carried out very cautiously causing high scrap rates and low manufacturing throughputs.
To alleviate the problems of limited adhesion to the substrate experienced with sol-gel type, vitreous coatings, the applicant has surprisingly discovered that reinforcing the internal side of the cooking item bottom with a discontinuous, hard enameled base makes it possible to significantly improve the adhesion of the non stick coating on metallic substrates (in particular, aluminum or aluminum alloy substrates), notably exhibiting an enhanced impact resistance of the non stick coating.
As used herein, what is meant by impact resistance is the ability of the coating to resist a violent shock.
By contrast, after a violent shock or impact) a coating having a low impact resistance exhibits, under the impact, a mark accompanied by radially propagating cracks. The adhesion in this area becomes so weak that the mere fact of scraping with a nail removes a portion of the coating whereof the size is actually higher than that of the impact itself.
More particularly, the object of the present invention is a cooking item comprising a metallic substrate having a concave internal side to be arranged on the side of the food, and a convex exterior side to be arranged on the heat source side, said internal side being successively coated from the substrate with a hard base layer and a non stick coating covering said hard base layer, wherein:                the hard base layer is a discontinuous enamel layer in the form of a superficial dispersion of enamel drops homogeneously distributed over the internal side of said item, the internal side being covered with a coverage ratio between 40% and 80%, a surface density between 300 drops/mm2 and 2000 drops/mm2 and a drop size between 2 μm and 50 μm, and        said anti stick coating is a PTFE-free, vitreous type coating shaped as a continuous film having a thickness of at least 10 μm and composed of a sol-gel material comprising a matrix of at least a metal polyalkoxilate and at least 5% by weight of the coating total weight of at least one metal oxide dispersed in said matrix.        
As used herein, what is meant by enamel is enamel having a softening point lower than the melting temperature of the substrate.
For instance, in the case of an aluminum or aluminum alloy substrate with a melting temperature of about 600° C., the softening point of enamel is lower than this temperature.
As used herein, what is meant by superficial dispersion of enamel drops is a discontinuous enamel layer in a divided state over a substrate (in this case, the substrate of the cooking item), such that the roughness of this layer is produced by the dispersed enamel drops.
As used herein, what is meant by coverage ratio of the substrate is the ratio, expressed in percent, of the substrate area actually covered by the superficial dispersion of enamel drops by the total area of the substrate which may be covered by the hard base layer.
As used herein, what is meant by surface roughness Ra, is the arithmetic mean deviation between the grooves and peaks of the surface with respect to the median (or mean) line, this deviation being assessed based on ISO 4287 standard.
It is observed that the presence of such a discontinuous enameled hard base layer arranged between the substrate and the sol-gel type non stick coating yields to a significant improvement of the impact resistance of the non stick coating.
With more particular regard to the enameled and discontinuous hard base layer, it exhibits a surface roughness Ra which actually depends on that of the substrate internal side whereon it is deposited.
Thus, in the case of a substrate whereof the internal side has been sand blasted prior to the deposition of the hard base layer and which exhibits a surface roughness Ra between 3 and 8 μm, the enameled and discontinuous hard base layer has then a surface roughness Ra comprised between 9 and 12 μm.
On the contrary, in the case of a substrate whereof internal side is smooth, the enameled and discontinuous hard base layer by which it is covered thus exhibits a surface roughness Ra comprised between 2 and 4 μm.
Advantageously, the enameled and discontinuous hard base layer has a hardness higher than or equal to that of the metal and/or metal alloy composing the substrate.
Now relating to the coating covering the structured internal side, and particularly, the sol-gel material composing this coating, this material matrix may advantageously comprise the condensation products of metal polyalkoxilates, for example, one or more polyalkoxisilanes, an aluminate, a titanate, a zirconate, a vandate, a borate, or mixtures thereof.
Preferably, the coating matrix according to the invention comprises a polyalkoxisilane and/or an aluminate such as to compose a mixed matrix.
In an alternative embodiment of the invention, the coating matrix according to the invention is grafted through one or several organic groups selected from C1-C4 alkyl groups and phenyl groups. These groups are required for improving the hydrophobic capability of the coating. In view to obtaining a better thermal stability for the coating, shorter chains are preferred in the frame of the present invention.
Preferably, the coating matrix according to the invention is grafted through one or more methyl groups, which enhance the hydrophobic feature of the coating without interfering with the formation of the inorganic network.
Besides the matrix of at least one metal polyalcoxylate, the vitreous coating according to an embodiment of the invention, comprises at least 5% by weight, and preferably from 5% to 30% by weight of the total weight of the coating, of at least one metal oxide, which is preferably finely dispersed in the matrix. This metal oxide is of a generally colloidal shape in the form of aggregates, whose size is lower than the micron, or even from 300 to 400 nm.
As colloidal metal oxide for use in the non stick coating according to the invention, silica, alumina, cerium oxide, zinc oxide, vanadium oxide and zirconium oxide may be cited. Preferred colloidal metal oxides are silica and alumina.
The presence of a metal oxide in the coating matrix an embodiment of the invention makes it possible to obtain a film with sufficient thickness, that is, a thickness of at least 10 μm. if the coating thickness is lower than 10 μm, the mechanical strength of the formed film would not be sufficient.
Preferably, the film has a thickness comprised between 10 and 80 μm, and more preferably between 30 and 50 μm such that the film layer thus formed is continuous, consistent and sufficient to absorb the substrate roughness.
Advantageously, the sol-gel material composing the non stick coating may further comprise at least silicone oil in order to enhance the hydrophobic nature of the coating surface, and particularly following a thermal shock such as flame exposure type.
In this regard, the metal polyalkoxylate has hydrophobic groups which are destroyed at high temperature during the passage over a flame. Nevertheless, this disappearance of the hydrophobic nature is temporary, as it is compensated for by the silicone oil trapped in the polyalkoxylate and whereof surface migration in infinitesimal amounts promotes the progressive rebuild of hydrophobic groups on the film surface.
With a coating according to an embodiment of the invention comprising at least 0.1% by weight of silicone oil, it is observed that the rebuild of the hydrophobic nature is sufficient at the time of a new cooking. In this regard, the value of the static contact angle θ of a water drop deposited on the coating of the invention is of about 20° after a flame exposure type thermal shock. This static contact angle value goes up to at least 75° after a process of rebuilding the hydrophobic features of reheating the room temperature at 200° C., for a period of at least 5 minutes, that is, when the item is ready for a new cooking.
Preferably, the silicone oil represents 0.1 to 6% by weight and more preferably 0.3 to 5% by weight of the total weight of the coating (dry state). Less than 0.1% by weight of silicone oil, the rebuild of hydrophobic groups that have disappeared during flame passage (600° C.) is lower, the obtained angle being lower than 62° C.
More preferably, the sol-gel material of the coating according to an embodiment of the invention comprises 0.5 to 2% by weight of silicone oil with respect to the total weight of the dry coating. In this case, the initial static contact angle θ of a water drop deposited on such a coating is of 95° C. Following flame passage type a thermal shock, this coating exhibits an angle of 20° C. Following a rebuild process comprising at least a step of reheating the room temperature to 200° C. over a period of at least 5 minutes, the static contact angle becomes higher than 75° C. when the item is ready for a new cooking.
The coating according to an embodiment of the invention may comprise a silicone oil or a mixture of silicone oils.
As silicone oils that may be used in the coating according to an embodiment of the invention, may be particularly cited phenyl silicones, methyl-phenyl silicones and methyl silicones.
If the coating according to an embodiment of the invention is to be in contact with food items, food grade silicone oil should preferably be selected, and in particular, one selected from food grade methyl-phenyl silicones and methyl silicones.
As methyl-phenyl silicone oils, may be particularly cited non food oils from WACKER under trade name WACKER SILICONOL AP150 and from DOW CORNING under trade name DOW CORNING 550 fluid, as well as food oils from WACKER ARM. As methyl silicone oils, oil from RHODIA under trade name RHODIA 47 V350, oil WACKER 200 fluid from WACKER or TEGO ZV 9207 oil from TEGO, may be cited, which are food-grade methylsilicone oils.
Preferably, a silicone oil selected from the aforementioned oils is used, which oil has a molecular weight of at least 1000 g/mol, which is non reactive and exhibits a viscosity between 20 et 2000 mPa·s.
Advantageously, the sol-gel material of the coating according to the invention may further comprise fillers for enhancing the mechanical features of the formed coating, and/or pigments, to render the coating colored. Moreover, the presence of fillers and/or pigments has also an advantageous effect on the hardness of the film.
As fillers which may be used in the coating according to an embodiment of the invention, alumina, zirconium, mica, clay (such as montmorillonite, sepiolite, gypsite, kaolinite and laponite®) and zirconium phosphate may be cited.
As pigments which may be used in the coating according to an embodiment of the invention, titanium dioxide, copper-chromium-manganese mixed oxides, iron oxide, carbon black, pyralene red, aluminosilicates, metal flakes and particularly aluminum flakes, may be cited.
Preferably, the fillers and/or pigments are flake shaped, which advantageously improves the hardness of the non stick coating.
Preferably, the fillers and/or pigments are nanoscaled so as to improve their dispersion and their distribution in the coating, providing the coating a high performance consistency.
In an advantageous version of the cooking item according to an embodiment of the invention, the substrate is a hollow cap of a cooking item, having a bottom and a lateral wall rising from said bottom.
The substrate of the cooking item according to an embodiment of the invention is advantageously made from a material selected from metals, glass and ceramic.
Metal substrates, and preferably anodized or non anodized aluminum or aluminum alloy, stainless steel, cast iron, iron or copper substrates are recommended.
As aluminum alloys that may be used to make the substrate of the cooking item according to an embodiment of the invention, are particularly recommended low alloyed, enamelable aluminum alloys, and in particular:                99% aluminum-containing “pure” aluminums of series 1000, and for example, alloys 1050, 1100, 1200 and 1350,        aluminum and manganese alloys of series 3000, and for example, alloys 3003, 3004, 3105 and 3005,        aluminum and silicon alloys of series 4000,        aluminum and magnesium alloys of series 5000, and for example alloys 5005, 5050 and 5052, and        aluminum, silicon and magnesium alloys of series 6000, and for example, alloys 6053, 6060, 6063, 6101 and 6951, and        aluminum, iron, silicon alloys of series 8000, and for example, alloy 8128.        
Finally, embodiments of the present invention also relate to a method for manufacturing a cooking item, comprising the steps of:
a) a step of providing a substrate having the end shape of the cooking item with a concave internal side to be disposed on the side of the food to be introduced in said item, and a convex exterior side to be disposed on the heat source side;
b) optionally, a step of treating the internal side of the substrate, so as to obtain a treated internal side adapted to cause a hard base layer to adhere on the substrate;
c) a step of preparing an adhering hard base layer on the substrate internal side, be it treated beforehand or not;
d) a step of preparing a non stick coating on the hard base layer formed in step c); the method including:                step c) of preparing a discontinuous, enameled hard base layer comprising the following successive steps of:                    c1) preparing an aqueous slurry of enamel frit;            c2) applying, through air spraying, the slurry on the substrate internal side with a pressure equal to or higher than 4 bars such that the amount of deposited slurry is comprised between 0.07 g/dm2 and 0.2 g/dm2; then            c3) drying and/or curing said enamel layer so as to form a hard biscuit of enamel or a layer of cured enamel; and                        step d) of preparing the non stick coating on said enamel hard biscuit or said cured enamel layer comprising the following successive steps of:                    d1) preparing a sol-gel composition (A+B) comprising at least a colloidal metal oxide and at least a metal oxide type precursor;            d2) applying over the entirety or over a portion of the enamel hard biscuit or said cured enamel layer at least a layer of the sol-gel composition (A+B) whose wet-basis thickness is at least of 20 μm; then            d3) curing said sol-gel composition layer (A+B) so as to obtain a non stick vitreous coating whose thickness is at least of 10 μm.                        
Advantageously, the enamel frit has less than 50 ppm of cadmium and less than 50 ppm of lead, and comprising 30 to 40% by weight of silica and 15 to 30% by weight of titanium oxide, less than 10% by weight of vanadium oxide and less than 4% by weight of lithium oxide with respect to the total weight of the frit, and the aqueous slurry comprises at least 20% by weight of mineral fillers with respect to the total weight of the slurry.
With regard to the preparation of the hard base layer on the substrate internal side, according to embodiments of the present invention, no homogeneous compound whose post-curing structure is determined by its pre-curing, initial chemical composition is applied, by contrast to an electric-arc or plasma projection method. In this regard, in embodiments of the present invention, an aqueous slurry of enamel frit is applied and it is observed during drying and/or curing that a homogenization of the different fusible elements of the slurry, those arising from the enamel frit and those arising from the slurry formulation thereof, takes place. Thus, in the present invention, there is no one-for-one correspondence between the slurry composition and the structure of the enamel formed following the application and the curing of this slurry.
The method pursuant to embodiments of the present invention has the advantage of implementing an aqueous slurry of an enamel frit which does not contain any solvent and thus generates no VOC, and, in embodiments, the enamel frit used in the method according to the present invention contains almost no noxious matter such as lead or cadmium or hardly in the form of little traces (50 ppm at the most of a noxious element), such that the enamel thus formed conforms to the food legislation in terms of enamel frit formulation or in terms of slurry formulation.
According to an alternative embodiment of the method of the invention, during step c) of preparing a discontinuous, enameled hard base layer, the non cured enamel layer is merely dried at a temperature comprised between 120° C. and 200° C. to yield an enameled biscuit.
According to a second alternative embodiment of the method of the invention, during step c) of preparing a discontinuous, enameled hard base layer, the non cured enamel layer is subjected to curing at a temperature comprised between 500 and 580° C.
With regard to the preparation of the sol-gel type non stick coating, in embodiments, the sol-gel composition A+B is prepared as follows:
i) preparing an aqueous composition A comprising 5 to 30% by weight of the total weight of the aqueous composition A of at least a metal oxide, and 0 to 20% by weight of the total weight of composition A of a solvent comprising at least one alcohol; then
ii) preparing a solution B comprising at least a metal alkoxide type precursor; then
iii) mixing solution B of metal alkoxide with aqueous solution A so as to obtain a sol-gel composition (A+B) containing 40 to 75% by weight of the aqueous composition A to the total weight of the sol-gel composition (A+B), so that the amount of colloidal metal oxide represents 5 to 30% by weight of the sol-gel composition (A+B) on a dry basis.
With more particular regard to the preparation of the aqueous composition A, the introduction of at least 5% by weight of at least a metal oxide with respect to the total weight of composition A is required to form a film whose post-curing thickness is at least of 10 microns. If, on the contrary, the amount is higher than 30% by weight of the total weight of composition A, composition A is not stable.
The metal oxide of the aqueous composition A is as defined above. Preferably, it is a colloidal metal oxide selected from colloidal silica and/or colloidal alumina.
The presence of an alcohol-based solvent is optional, but has the advantage of improving the compatibility of aqueous composition A with metal alkoxide solution B.
Nevertheless, it is also possible not to use a solvent, yet, in this case, the number of possible polyalkoxylates is reduced to those having an excellent compatibility with water. It is possible to have an excess amount of solvent (higher than 20%), yet, this unnecessarily generates non environmental-friendly volatile organic compounds.
As a solvent, an oxygenated alcohol-based solvent or ether-alcohol is preferably used in aqueous composition A.
Aqueous composition A according to embodiments of the invention may also comprise, besides the colloidal metal oxide, and, if need be, the alcohol-based solvent, at least a silicone oil, which is preferably present in composition A in an amount of 0.05% to 3% by weight with respect to the total weight of the composition.
With an aqueous composition A comprising 0.5 to 2% by weight of silicone oil, a coating exhibiting hydrophobic features which may rebuild in the frame of a cooking use. Silicone oil of composition A is a food-grade silicone oil defined above.
Aqueous composition A in embodiments of the invention may further comprise fillers and/or pigments such as defined above.
Aqueous composition A in embodiments of the invention may further comprise pyrogenic silica, which is intended to adjust the viscosity of the sol-gel composition and/or the glossy effect of the dry coating.
With regard to the preparation of solution B, as a precursor is preferably used a metal alkoxide selected from the group consisting in:                precursors whose chemical formula is M1(OR1)n,        precursors whose chemical formula is M2(OR2)(OR2)(n-1)R2′,        precursors whose chemical formula is M3(OR3)(OR2)(n-2)R3′2,        
R1, R2, R3 or R3′ being an alkyl group,
R2′ designating an alkyl or phenyl group,
n being an integer corresponding to the maximal valence of metals M1, M2, or M3,
M1, M2, or M3 designating a metal selected among Si, Zr, Ti, Sn, Al, Ce, V, Nb, Hf, Mg or Ln,
Advantageously, the metal alkoxide of solution B is an alkoxysilane.
As alkoxysilanes which may be used in solution B of the method of the invention, methyltrimethoxysilane (MTMS), tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), dimethyldimethoxysilane, and mixtures thereof may be cited.
Preferably, alkoxysilanes MTES and TEOS will be used as they do not contain methoxy groups. In this regard, the hydrolysis of methoxy leads to the formation of methanol in the sol-gel formulation, which, due to its rating as a toxic element, requires additional precautions during application. On the contrary, the hydrolysis of ethoxy groups generates only ethanol having a more favorable rating and hence, less restricting usage prescriptions with regard to the sol-gel coating.
According to an advantageous embodiment of the method of the invention, solution B may comprise a mixture of an alkoxylane such as defined above as well as an aluminum alkoxide.
The metal alkoxide type precursor of solution B is a mixture of a mineral, organic Lewis acid representing 0.01 to 10% by weight of the total weight of solution B.
As acids which may be used for mixing with the metal oxide precursor, acetic acid, citric acid, ethyl aceto-acetate, hydrochloric acid or formic acid may particularly be cited.
Preferred acids of embodiments of the invention are organic acids, and more particularly, acetic acid and formic acid.
After preparing aqueous composition A and that of precursor solution B, they are mixed together, in order to form a sol-gel composition (A+B). The respective amounts of each of compositions A and B should be adjusted such that the amount of colloidal silica in the sol-gel composition represents 5 to 30% by dry weight.
The sol-gel composition (A+B) of embodiments of the invention may be applied over the substrate through spraying or through any other application mode, such as by soaking, stamping, with a paintbrush, with a roller, by spin-coating or serigraphy. Nevertheless, in the frame of a shaped object, for example, gun spraying has the advantage of forming a continuous, homogenous film which forms, after curing, a continuous coating whose thickness is smooth and tight.
Generally, after applying the sol-gel composition (A+B) according to embodiments of the invention, drying is carried out preferably at a temperature of 60° C. for a period of 1 minute.
The nature of the non stick coating changes according to the curing temperature, from an organo-mineral coating, for a curing temperature of about 200° C. to a substantially mineral coating for higher curing temperatures.
At a curing temperature lower than 400° C., particularly between 180 and 350° C., the non stick coating is an organo-mineral coating (unless the precursor is solely a TEOS, in this case, a substantially mineral coating would be produced, even in the case of curing temperatures lower than 400° C.).
Advantageously, a hollow cap, such as described previously, with a bottom and a lateral wall rising from the bottom should be used as the substrate to be used to prepare the cooking item according to embodiments of the invention.
The substrate to be used in the frame of the present invention will advantageously be made from a material selected from metals, glass, and ceramics.
Advantageously, anodized or non anodized, aluminum or aluminum alloy substrates, stainless steel, cast iron, iron or copper substrates may be cited as the metal substrates which may be used in embodiments of the method of the invention.
The method according to embodiments of the invention may further comprise a step of depositing an enamel layer on the side opposite to that coated with a non stick coating according to the invention, this enamel layer deposition step being carried out prior to the non stick coating step according to embodiments of the invention.
Besides the aforementioned advantages, the implementation of the method in accordance with embodiments of the invention is particularly simple, and this method may be considered without introducing important modifications to usual manufacturing processes of cooking item cooking items.
Other advantages and features of the present invention will become more apparent from the following description, given only by way of non limitative example and made with reference to the accompanying drawings.